Layered deposition bridge tooling

ABSTRACT

Disclosed is a method for making a prototype plastic injection molded part from a mold tool ( 10 ) built by fused deposition modeling. The mold too ( 10 )l is built by depositing roads of a molten thermoplastic resin in layers in a predetermined pattern defined by computer file data representing the inverse of the desired prototype molded part, and is used in an injection molding machine without the addition of any reinforcement fill material or layers to create the prototype part. The disclosed method provides prototype plastic injection molded parts within a twenty-four hour time period.

CROSS-REFERENCE TO RELATED APPLICATION(S) BACKGROUND OF THE INVENTION

The present invention relates to prototyping of injection moldedobjects, and more particularly to methods for rapidly making mold toolsfor use in plastic injection molding prototyping processes.

In a typical injection molding process, plastic is injected at highpressures, extremely quickly, into a thermally conductive metal mold.The molded part is quickly cooled to a temperature at which it can beremoved from the mold. The part is then quickly ejected from the mold sothat another part can be made, and so that the part does not becomestuck on the mold (due to shrink differential). Cooling of large partscontinues on a fixture. The goals of production injection modeling areto produce a high quantity of high-quality parts in a short turn-aroundtime. A thirty second cycle time or less for the making of each moldedpart is typical.

In order to produce a three-dimensional object in a typical injectionmolding process, it is necessary to prepare a mold tool that has acavity which is complementary to the desired shape of thethree-dimensional object. The mold tool generally consists of twoopposing halves, which mate together to define the mold cavity. The moldtool is normally machined out of steel or other metal which is capableof withstanding high temperature and pressure when hot liquid isinjected into the mold. In use, the mold tool is inserted into a frameof an injection molding machine, and held in place with high clampingforces to oppose pressure generated inside the mold. The time and skillrequired to prepare the mold tool are both significant. The machiningmust be done by skilled craftsmen, and includes the incorporation of asprue through which the molding material is injected, a vent, coolinglines and ejector pins. Typically, this process involves placing anorder with an outside vendor and waiting several weeks or months fordelivery, at high cost.

Before undergoing the expense and long lead time associated withconventional metal mold manufacturing, it is desirable to produce aprototype of the part that will have similar characteristics to theproduction part. The goal is produce a prototype having characteristicssufficiently close to that of the desired final manufactured part so asto permit a close prediction of part performance. Various additiveprocess rapid prototyping (RP) technologies are commonly used to makeprototype parts in the design stages of a part. These rapid prototypingtechnologies include fused deposition modeling (FDM), stereolithography(SLA), selective laser sintering (SLS), laminated object manufacturing(LOM) and jet technology. These additive process techniques produceprototypes useful for evaluating the fit, form and function of a partdesign, to gain preliminary part approval and to accelerate productdevelopment. The strength of a final production part is not, however,replicated in prototypes created by these rapid prototyping techniques.The additive processes create layers, layered stress points and voids inthe part resulting in a different internal stress structure than that ofthe homogeneous injection-molded part. Additionally, many materials usedin these processes are weak.

Various methods have been developed for creating mold tools used to makeprototype injection molded parts, which may be referred to as “bridgetooling” or “temporary tooling.” A number of these methods utilize rapidprototyping techniques, particularly, stereolithography. For example,U.S. Pat. No. 5,439,622 describes the use of stereolithography to form amold shell, which is then reinforced with an incompressible material andcoated with a thermally conductive material. U.S. Pat. No. 5,989,679describes a mold tool formed by injecting a strengthening material intocavities within an object formed by stereolithography. U.S. Pat. No.5,952,018 describes a mold tool, including an ejection valve within themold tool, formed by stereolithography. U.S. Pat. No. 5,641,448describes the making of a mold tool by depositing a metal coating onto aplastic mold shell produced by stereolithography.

The use of rapid prototyping to create molds for use in processes otherthan injection molding are also known. For example, U.S. Pat. No.6,073,056 describes a mold built by stereolithography or fuseddeposition modeling used to form a vacuum cast part. U.S. Pat. No.6,103,156 describes the making of a prototype part by pouring athermoset into a mold formed by a rapid prototyping technique.

Techniques are also known which use a part formed a rapid prototypingprocess as a master mold pattern to create a prototype mold tool. Forexample, U.S. Pat. No. 5,189,781 describes the use of a prototype partas the pattern for making a sprayed metal mold. U.S. Pat. No. 5,707,578uses a prototype created by stereolithography as a master mold.

A commercial process known as the Swiftool™ process uses a prototypepart, which may be made by a rapid prototyping technique, as a patternfor creating bridge tooling. The process takes several days. Anothercommercial process known as 3D Keltool® makes bridge tooling in a periodof several days in a metal-powder sintering process, starting from amaster pattern made by stereolithography. Yet another commercial systemcalled AIM™ builds mold tools by stereolithography using UV-sensitivematerials.

While the above-described methods do reduce the time and expense ofmaking mold tools, such methods nonetheless require finishing stepswhich can be tedious and which require additional time and skill tocomplete. There is a need for a more rapid and low cost method of makinga mold tool which can be used to create a small number of prototypeinjection molded parts.

BRIEF SUMMARY OF THE INVENTION

The present invention is a method for making a prototype plasticinjection molded part using a mold tool made by a fused depositionmodeling technique. In one embodiment, the mold tool is built in two ormore portions, wherein layers of thermally solidifiable material aredeposited in a predetermined pattern according to computer file datarepresenting the mold shape. Each mold portion includes a mold surface,a mating surface, and a base which supports the mold and matingsurfaces. Together the mold portions define a mold cavity. A spruechannel and alignment holes are either formed into the mold tool as itis built, or machined into the mold tool after it is built. A ventchannel may likewise be built or machined into the mold tool, or, thebuild process itself may be designed to result in the mold tool itselfhaving a porosity sufficient to vent the tool. Optionally, the moldsurfaces and mating surfaces may be smoothed by a vapor smoothingprocess to remove unintentional ridges in the surfaces. The mold tool isused in an injection molding machine, without the addition of anyreinforcement fill material or layers, to create the prototype part.

In an alternate embodiment, the mold tool is made from a solublemodeling material and has a single-piece construction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of two mold portions of an exemplary mold toolproduced by fused deposition modeling in accordance with the presentinvention.

FIG. 2 is a sectional view of the mold portions of FIG. 1, taken along aline 2-2 of FIG. 1 and mated together to define a mold cavity.

FIG. 3 is a flow diagram of the process of making a prototype injectionmolded part using a mold tool built in accordance with the presentinvention.

DETAILED DESCRIPTION

FIG. 1 shows two halves of an exemplary mold tool 10 built in accordancewith the present invention. A first portion 12 of mold tool 10 includesa recessed mold surface 14 corresponding to the shape of a first half ofa desired prototype molded part. A second portion 16 of mold tool 10includes a recessed mold surface 18 corresponding to the shape of asecond half of the desired prototype molded part. The mold portions 12and 16 each have a mating surface 17 and a base 20 shown in FIG. 2,which supports the mold surfaces 14 and 18 and the mating surfaces 17.When the mating surfaces 17 of the mold portions 12 and 16 are matedtogether as shown in FIG. 2, the mold surfaces 14 and 18 define a moldcavity 19, which has the shape of the desired prototype part. Forprototype molded parts that have interior cavities, the mold tool 10further comprises a mold core.

The mold portions 12 and 16 each also include a sprue channel 22, a ventchannel 24, and four alignment holes 26. The sprue channels 22 allow forthe placement of a sprue which will be inserted in a final assembly ofthe mold tool 10, providing a path for the injection of molten plasticinto the mold cavity 19. The vent channels 24 together form a passagefor the venting of gas from the mold cavity 19 when the mold tool 10 isassembled.

The alignment holes 26 receive screws or pins, which align and holdtogether the mold tool portions 12 and 16 in assembly of the mold tool10. The mold tool 10 may also optionally include cooling lines forintroducing a flow of coolant during an injection process.

In an alternate embodiment, a mold tool is made from a soluble modelingmaterial and has a single-piece construction. The soluble materialpermits a single-piece construction, as the molt tool may be dissolvedfrom a prototype part after the part is formed. In contrast, a mold toolmade from an insoluble material is removed from a prototype part bymechanically disengaging the mold portions. A suitable soluble modelingmaterial is an alkali-soluble material comprising a base polymercontaining a carboxylic acid, and a plasticizer. The base polymercomprises a first comonomer (which contains carboxylic acid) and asecond comonomer that is polymerized with the first comonomer to providethermal and toughness properties suitable for fused deposition modeling.A preferred base polymer is comprised of methacrylic acid as the firstcomonomer and an alkyl methacrylate (e.g., methyl, ethyl, propyl orbutyl methacrylate, and combinations thereof), preferably methylmethacrylate, as the second comonomer. A desirable amount of theacid-containing first comonomer is 15-60 weight percent of the basepolymer. The base polymer is plasticized to attain rheologicalproperties desired for the modeling process. Most preferably, thealkali-soluble thermoplastic material contains between about 84 weightpercent and 74 weight percent of the base polymer and contains betweenabout 16 weight percent and 26 weight percent of the plasticizer, andhas a melt flow index of between about 5 g/10 minutes and 10 g/10minutes under a load of 1.2 kg at 230° C. A mold tool made from thealkali-soluble material is removed from the prototype part by placingthe mold tool containing the part in an alkaline bath. Thealkali-soluble modeling material is the subject of co-pending U.S.patent application Ser. No. 10/019,160, International Application No.PCT/US00/10592 (published as WO 00/62994), assigned to the same assigneeas the present application, and which is hereby incorporated byreference as it set forth fully herein.

The mold tool of the present invention is built by a fused depositionmodeling process. Fused deposition modeling is a rapid prototypingtechnique that builds up three-dimensional objects in layers byextruding molten modeling material in a predetermined pattern accordingto computer file data representing the mold tool. The computer file datais derived from information available on the desired prototype moldedpart. For example, typically, the part is designed using acomputer-aided design (CAD) system, and corresponding informationrelating to the outline of the part is derivable from a CAD filedefining the desired part. A computer program designs the mold portionsin accordance with the outline of the desired part, as the inverse ofthe desired part shape. A software program available from MoldflowCorporation will design the mold portions in this manner. A furthersoftware program “slices” the computer representation of the moldportions into horizontal layers. The modeling machine extrudes the roadsof modeling material layer-by-layer, with each extruded road having athickness equal to the height of a slice. The extruded material fuses topreviously deposited material and solidifies upon a drop in temperatureto form the mold portions. The mold portions may be built simultaneouslyin the modeling machine, or one at a time. In a preferred embodiment,the mold portions 12 and 16 are built from a polyphenylsulfone resin ona Stratasys® Titan™ FDM® fused deposition modeling machine.

The sprue channels 22, the vent channels 24, the alignment holes 26, andany cooling lines are preferably formed into the mold portions 12 and 16as they are built. This can be done by including such features in thecomputer file data representing the mold tool 10. Alternatively, a spruechannel, vent channel, cooling lines and/or alignment holes may bemachined into the mold portions 12 and 16 after they are built. Thechannels 22 and 24 and the alignment holes 26 shown in the exemplarymold tool 10 are merely one example of the placement and design of suchfeatures. Alternative designs include vertical orientation of thechannels 22 and 24, and forming a single sprue channel or vent channelwithin one or the other of mold portions 12 and 16.

The need for a vent channel in the mold tool 10 may be avoided bycontrolling the extrusion pattern of the roads so that the mold tool 10has an inherent porosity providing an open-cell matrix sufficient tovent gas from the mold cavity 19. Controlled porosity fused depositionmodeling is taught in U.S. Pat. No. 5,653,925.

The mold tool 10 is formed from a thermoplastic resin that is compatiblewith the fused deposition modeling process and that will sustain thetemperature and pressure of the injection molding process, so as toproduce at least one prototype plastic injection molded part. Anexemplary thermoplastic resin comprises at least 50 weight percent of athermoplastic selected from the group consisting of polyphenylsulfone,polysulfone, polystyrene, polyphenylene ether, amorphous polyamides,polycarbonate, polyaryletherketone, acrylics (e.g., methylmethacrylate), nylon, poly(2-ethyl-2-oxazoline), and blends thereof. Thethermoplastic resin may contain various fillers, additives and the like,as will be understood by those skilled in the art. A particularlypreferred thermoplastic for use in creating a mold tool in accordancewith the present invention is a polyphenylsulfone-based resin. One suchthermoplastic comprises polyphenylsulfone blended with between about2-20 weight percent polycarbonate (preferably near 10 percent).

FIG. 3 shows a flow diagram which summarizes the method of producing aprototype injection molded part in accordance with the presentinvention. A CAD tool is used to generate computer file datarepresenting a mold tool, in step 40. The data is provided to a fuseddeposition modeling machine, in step 42. The mold tool is built in thefused deposition modeling machine, in layers defined by the computerfile data, in step 44. In an optional step 46, the mold surfaces and/ormating surfaces of the mold tool are smoothed to remove ridgesunintentionally created in the formation of the mold tool. In apreferred embodiment, the smoothing is done by a vapor smoothingprocess, which is the subject of International Application No.PCT/US03/______ entitled “Smoothing Method For Layered DepositionModeling”, W. Priedeman and D. Smith, filed on even date herewith,assigned to the same assignee as the present application, andincorporated by reference as if set forth fully herein. As is taught insaid co-pending application, certain mold features may be identified forsolvent masking or for pre-distortion prior to the vapor smoothing step,and the computer file data representing the mold tool may include dataidentifying said features. Alternative smoothing techniques includesanding, grinding, and thermal ironing.

The mold surfaces of the mold tool are then coated with a release agent,in a step 48. Suitable release agents include dry film lubricants, andothers that will be recognized by those skilled in the art. If needed,sprue and vent channels and alignment holes are machined into the moldtool prior to step 48. A final step 50 is to perform injection moldingusing the mold tool. The mold tool is assembled in an injection moldingmachine, without the addition of any reinforcement fill material orlayers.

Using the method of the present invention, a prototype plastic injectionmolded part can be produced within a 24-hour time period.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

1. A method for making a prototype plastic injection molded part,comprising the steps of: providing computer file data representing amold tool; building the mold tool by depositing roads of a moltenthermoplastic resin in layers in a predetermined pattern defined by thecomputer file data, the mold tool defining a mold cavity; and using themold tool in an injection molding machine, without the additional of anyreinforcement fill material or layers, to create the prototype part byinjection molding of plastic.
 2. The method of claim 1, wherein thethermoplastic resin comprises at least about 50 weight percent of athermoplastic selected from the group consisting of polyphenylsulfone,polysulfone, polystyrene, polyphenylene ether, amorphous polyamides,polycarbonate, acrylics, nylon, poly(2-ethyl-2-oxazoline), and blendsthereof.
 3. The method of claim 1, wherein the thermoplastic -resin is apolyphenylsulfone-based resin.
 4. The method of claim 1, wherein a spruechannel and alignment holes are formed into the mold tool as it isbuilt.
 5. The method of claim 1, and further comprising the step of:machining a sprue channel into the mold tool, prior to the step of usingthe mold tool in an injection molding machine.
 6. The method of claim 1,and further comprising the step of: machining a plurality of alignmentholes into the mold tool, prior to the step of using the mold tool in aninjection molding machine.
 7. The method of claim 1, wherein thepredetermined pattern results in the mold tool having a porositysufficient to vent gas in the mold cavity generated by injection of theplastic.
 8. The method of claim 1, wherein a vent channel is formed intothe mold tool as it is built.
 9. The method of claim 1, and furthercomprising the step of: machining a vent channel into the mold tool,prior to the step of using the mold tool in an injection moldingmachine.
 10. The method of claim 1, and further comprising the step of:coating surfaces of the mold cavity with a release agent prior to thestep of using the mold tool in an injection molding machine.
 11. Themethod of claim 1, wherein the step of building the mold tool comprisesbuilding two or more mating mold portions, each mold portion having amold surface, a mating surface, and a base which supports the mold andmating surfaces, the mold surfaces together defining the mold cavity.12. The method of claim 11, and further comprising the step of:smoothing surfaces of the mold tool prior to the step of using the moldtool in an injection molding machine.
 13. The method of claim 11,wherein the step of building the mold tool further comprises building amold core.
 14. The method of claim 13, and further comprising the stepof: assembling the mold core in the mating portions of the moldtool-prior to the step of using the mold tool in an injection moldingmachine.
 15. The method of claim 14, wherein the thermoplastic resinforming the mold core is an alkali-soluble thermoplastic, comprising abase polymer containing between about 15 weight percent and 60 weightpercent of a carboxylic acid, and a plasticizer, and further comprisingthe step of: dissolving the mold core from the prototype part.
 16. Amethod for making a prototype plastic injection molded part, comprisingthe steps of: providing computer file data representing a mold tool;building the mold tool by depositing roads of a molten solublethermoplastic resin in layers in a predetermined pattern defined by thecomputer file data; using the mold tool in an injection molding machine,without the additional of any reinforcement fill material or layers, tocreate the prototype part by injection molding of plastic; anddissolving the mold tool to release the prototype part.
 17. The methodof claim 16, wherein the soluble thermoplastic resin is analkali-soluble thermoplastic comprising: a base polymer containingbetween about 15 weight percent and 60 weight percent of a carboxylicacid, and a plasticizer.
 18. The method of claim 17, wherein thecarboxylic acid is rnethacrylic acid and wherein the base polymerfurther contains an alkyl methacrylate.
 19. The method of claim 18,wherein the alkyl methacrylate is methyl methacrylate and wherein thebase polymer contains between about a 1:1 to a 1:2 weight percent ratioof methacrylic acid to methyl methacrylate.